This invention relates in general to hydraulic bushings, and more particularly to hydraulic bushings tuned to reduce transmission of vibrations therethrough.
A typical application for hydraulic bushings (or mounts) is to locate them between components where a reduction in transmitted vibrations and/or the vibration of one of the components should be damped in a specific frequency range are desired. For example, engine mounts or suspension mounts in a vehicle, which are located between an engine and a vehicle chassis, or between a suspension and a vehicle chassis, respectively. A typical hydraulic bushing employed as a vehicle suspension mount includes an inner core connected to an inner support structure, commonly known as an inner ring, by an elastomeric material to form an assemblyxe2x80x94with the assembly being received in a housing. The housing is,typically mounted to a component of the chassis, while the core is typically mounted to a component of the suspension. The elastomeric material is typically all rubber since the rubber is an elastomeric material with good sealing properties for retaining hydraulic fluid and is easy to mold into a desired shape. The rubber portion includes cavities with channels extending between them. A hydraulic fluid is provided in the cavities and sealed-in by the rubber. The cavities, channels and fluid, in conjunction with the rubber, are designed (i.e. tuned) to damp a particular low frequency vibration.
With this bushing, then, when the suspension or chassis receives a vibration at the tuned frequency, the hydraulic fluid in the suspension mount that is displaced through the channel between the desired chambers is resonating in the channel and damps the vibration, thus reducing the vibration. This type of damping is most effective for low frequencies with relatively large amplitudes, for example, in the range of 10-40 hertz. However, under relatively high frequency excitation, the hydraulic fluid behaves more like a solid, which significantly increases the overall dynamic stiffness of the bushing. Consequently, it increases the transmissibility of the high frequency vibrations through the mount, which is detrimental to the intended functionality of the mount.
Thus, it is desirable to provide a hydraulic bushing that can be employed as an engine or suspension mount, with the bushing tuned to reduce vibrations at a given lower frequency, while still maintaining a relatively lower dynamic stiffness of the mount assembly under relatively high frequency oscillations.
In its embodiments, the present invention contemplates a hydraulic bushing adapted to form a mount and transfer load between a first component and a second component. The hydraulic bushing includes a core having a perimeter, and an inner support structure surrounding and spaced from the perimeter. The hydraulic bushing also includes a spring portion connected between the core perimeter and the inner support structure, with the spring portion including a first spring portion formed of rubber and defining hydraulic fluid cavities and a fluid channel extending between the cavities, with the cavities located about a first portion of the perimeter, and a second spring portion formed of microcellular polyurethane about a second portion of the perimeter, whereby the first spring portion and the second spring portion are located in parallel.
An embodiment of the present invention also contemplates a method of damping vibrations of a load transferred through a hydraulic bushing between a core and an inner support structure, the method comprising the steps of: transferring a first portion of the load through a first spring portion that is made of microcellular polyurethane; and transferring a second portion of the load through a second spring portion, with the second spring portion including rubber defining hydraulic fluid cavities and a fluid channel extending between the cavities, and a hydraulic fluid located in the hydraulic fluid cavities and the channel.
An advantage of the present invention is that the hydraulic bushing can be tuned to reduce transmitted vibrations at a relatively lower frequency, while still maintaining a lower dynamic stiffness at relatively higher frequencies than a hydraulic bushing with an all rubber support structure between the inner core and the inner support structure. That is, the present invention improves the vibration isolation at relatively higher frequencies.
The present invention is particularly advantageous at improving the vibration isolation when the relatively high frequency excitations have a relatively high amplitude.
Another advantage of the present invention is that microcellular polyurethane (MCU) can be used to isolate relatively high frequency vibrations while still allowing the rubber to seal in the hydraulic fluid.